Viscoplastic Self-consistent Code Research Articles

Strain localization at longitudinal weld seams during plastic deformation of Al–Mg–Si–Mn–Cr extrusions: The role of microstructure

The current work was motivated by the increasing use of hollow aluminum extruded profiles for automotive applications and the need for a better understanding of the mechanical response of extrusion seam welds. Two Al–Mg–Si alloys were studied, one without Mn and one with 0.5 wt%Mn and 0.15 wt%Cr. A high temperature homogenization was applied, resulting in a high density of α-Al(Fe,Mn)Si dispersoids in the Mn containing alloy, and an essentially dispersoid free microstructure in the other. The alloys were extruded through a simple porthole die (with two ports) to produce a strip with a centerline weld seam. The Mn free extrudate had a recrystallized grain structure whereas the Mn and Cr containing material was unrecrystallised. There were significant spatial variations of microstructure between the weld seam and surrounding regions, in particular the crystallographic texture. Tensile tests were conducted with the loading direction perpendicular to the weld seam using digital image correlation (DIC) to characterize the local strain distribution. Strain localization and final fracture occurred at weld seam for the unrecrystallized material whereas these occurred away from the weld line in the recrystallized case. The role of local crystallographic texture was examined through polycrystal plasticity simulations using the visco-plastic self consistent code (VPSC). These simulations indicated that the weld seam region was mechanically stronger than the surrounding region for the recrystallized alloy and correspondingly weaker for the unrecrystallized case, thereby rationalizing the experimental results of strain localization and final fracture location.

Thermal stability of microstructure, mechanical properties, formability parameters and crystallographic texture in an Al-7075 alloy processed by accumulative roll bonding

Accumulative roll bonding (ARB) is a severe plastic deformation (SPD) technique that has been extensively used in a wide range of metals to improve their mechanical properties. In particular, ARB processed materials display an increase in both yield and ultimate tensile strength, since the SPD processing induces a remarkable grain refinement. In precipitation-hardened alloys, such as aluminum 7075, this leads to a synergistic effect which further enhances strength. Although the reported results are promising for high-strength applications, thermal stability of the microstructure after processing remains an open question. In this paper, we present an investigation of the thermal stability of the texture, microstructure, and mechanical properties of Al-7075 samples processed by ARB to an equivalent Von Mises strain of 240%. After processing, yield and tensile strength displayed improvements of 30% and 20% respectively over the T6 condition. Crystallographic texture was found to depart from the rotated cube component to Dillamore/Taylor and Goss components. Several annealing heat treatments ranging from 373 to 673 K (100–400 °C) were carried out and it was found that the improved mechanical properties are stable up to 423 K (150 °C). The texture was found to slowly revert to a rotated cube with increasing temperature. The results are discussed in terms of the combination of strength and formability as calculated by the Lankford parameter using the visco-plastic self-consistent code (VPSC).

Efficient rolling texture predictions and texture-sensitive thermomechanical properties of α-uranium foils

Finite element (FE) analysis was used to simulate the strain history of an α-uranium foil during cold straight-rolling, with the sheet modeled as an isotropic elastoplastic continuum. The resulting strain history was then used as input for a viscoplastic self-consistent (VPSC) polycrystal plasticity model to simulate crystallographic texture evolution. Mid-plane textures predicted via the combined FE→VPSC approach show alignment of the (010) poles along the rolling direction (RD), and the (001) poles along the normal direction (ND) with a symmetric splitting along RD. The surface texture is similar to that of the mid-plane, but with a shear-induced asymmetry that favors one of the RD split features of the (001) pole figure. Both the mid-plane and surface textures predicted by the FE→VPSC approach agree with published experimental results for cold straight-rolled α-uranium plates, as well as predictions made by a more computationally intensive full-field crystal plasticity based finite element model. α-uranium foils produced by cold-rolling must typically undergo a recrystallization anneal to restore ductility prior to their final application, resulting in significant texture evolution from the cold-rolled plate deformation texture. Using the texture measured from a foil in the final recrystallized state, coefficients of thermal expansion and the elastic stiffness tensors were calculated using a thermo-elastic self-consistent model, and the anisotropic yield loci and flow curves along the RD, TD, and ND were predicted using the VPSC code.

Average intragranular misorientation trends in polycrystalline materials predicted by a viscoplastic self-consistent approach

This work presents estimations of average intragranular fluctuations of lattice rotation rates in polycrystalline materials, obtained by means of the viscoplastic self-consistent (VPSC) model. These fluctuations give a tensorial measure of the trend of misorientation developing inside each single crystal grain representing a polycrystalline aggregate. We first report details of the algorithm implemented in the VPSC code to estimate these fluctuations, which are then validated by comparison with corresponding full-field calculations. Next, we present predictions of average intragranular fluctuations of lattice rotation rates for cubic aggregates, which are rationalized by comparison with experimental evidence on annealing textures of fcc and bcc polycrystals deformed in tension and compression, respectively, as well as with measured intragranular misorientation distributions in a Cu polycrystal deformed in tension. The orientation-dependent and micromechanically-based estimations of intragranular misorientations that can be derived from the present implementation are necessary to formulate sound sub-models for the prediction of quantitatively accurate deformation textures, grain fragmentation, and recrystallization textures using the VPSC approach.

Plastic anisotropy and slip systems in ringwoodite deformed to high shear strain in the Rotational Drickamer Apparatus

High shear strain deformation experiments up to equivalent strains of 180% were performed on ringwoodite at conditions of 23GPa and 1800K. At very large shear strains, deviation in the strengths of the (311) and (400) lattice planes is observed, indicating plastic anisotropy. Lattice strain theory was applied to calculate effective viscosities of the available slip systems from the observed plastic anisotropy. We find that the effective viscosities (unitless) are proportional to 0.43, 0.46, and 1.54 for the {111}〈11¯0〉, {11¯0}〈110〉 and {001}〈11¯0〉 slip systems respectively. This indicates that {111}〈11¯0〉 slip is slightly softer than {11¯0}〈11¯0〉 at these conditions. Additionally the {111}〈11¯0〉 slip system has significantly more symmetric variants than the slip systems involving other lattice planes, thus it is expected to dominate deformation. This is confirmed by polycrystal plasticity modeling using the viscoplastic self-consistent code (VPSC) which indicates that simulations with dominant {111}〈11¯0〉 slip provide the best match to experimental textures.

Accounting for local interactions in the prediction of roping of ferritic stainless steel sheets

The effect of the spatial distribution of crystallographic orientations on roping amplitude and wavelength in ferritic stainless steel has been evaluated. The through-thickness mechanical behaviour of a sheet deformed in tension has been tested experimentally and simulated using a full-field viscoplastic fast Fourier transform formulation. These crystal plasticity simulations use orientation imaging microscopy data as input, allowing for large-scale simulation domains to be investigated while accounting for the clustering of orientations with similar deformation behaviour. The simulations predict both the local deformation response as well as the macroscopic surface roughness. The latter is compared quantitatively with experimental measurements and is shown to predict both the wavelength and amplitude of the observed roping. The results of these simulations have also been compared with previously proposed mean-field crystal plasticity simulations of roping, performed using the viscoplastic self-consistent code, in which each crystal orientation is, at most, influenced by the behaviour of a homogenized matrix, but not by its local neighbourhood. Comparison between these two kinds of approaches thus allows us to assess the significance of the local neighbourhood on the macroscopic prediction of roping.

Modeling and simulation of irradiation hardening in structural ferritic steels for advanced nuclear reactors

Hardening and embrittlement are controlled by interactions between dislocations and irradiation induced defect clusters. In this work we employ the visco plastic self consistent (VPSC) polycrystalline code in order to model the yield stress dependence in ferritic steels on the irradiation dose. We implement the dispersed barrier hardening model in the VPSC code by introducing a hardening law, function of the strain, to describe the threshold resolved shear stress required to activate dislocations. The size and number density of the defect clusters varies with the irradiation dose in the model. We find that VPSC calculations show excellent agreement with the experimental data set. Such modeling efforts can both reproduce experimental data and also guide future experiments of irradiation hardening.